Frequent inactivation of p16 and p15 expression in human esophageal squamous cell carcinoma detected by RT-PCR

Abstract

AIM: The cyclin-dependent kinase inhibitors p16 and p15 play important roles in the regulation of the cell cycle, and have been found to have tumor suppressor roles in a variety types of cancer. It has been shown that p16 aberrant methylation and p15 homozygous deletions were frequently involved in human esophageal squamous cell carcinoma (ESCC). The present study was to examine the impact of such molecular alterations on the expression of these genes.

METHODS: The mRNA level of both genes was measured in 21 frozen ESCC specimens using semi-quantitative RT-PCR.

RESULTS: Nineteen cases were observed at a low basal level of p16 expression (0.11 ± 0.07, expression units normalized by housekeeping glyceraldehyde-3-phosphate dehydrogenate gene as internal standard) in the normal epithelia adjacent to the cancer tissue. Among the 19 cases, only 5 showed a significant elevation of p16 expression ( > 3.2 folds) in the tumor, whereas the remaining 14 showed either a slight increased (1-2 folds), or decreased p16 expression compared to normal, whereas 11 had only a slight increase (1-2 folds) or a decrease in tumor. In the 5 cases where p15 was already activated ( > 0.5) in the adjacent normal epithelium, 4 of them had similar or a slightly lower expression level, but one had a great decrease in p15 expression ( < 1% of the normal level). For intact p16 and p15 genes, which encode cell cycle regulators, significant increase of their expression is expected in the cancer cells as a response to accelerated cellular proliferation. However, in our samples, significant activation was only seen in 7 cases for p16 gene and 9 cases for p15 gene. Fourteen cases for p16 (70%) and 12 cases (57% ) for p15 either maintained low basal levels or had decreased expression levels in tumor, respectively, which indicate suppression or inactivation of the genes. No correlation between p16 and p15 expression was observed in our frozen samples.

CONCLUSION: The present findings indicate that distinct and ind ependent mechanisms are involved in the inactivation of p15 and p16 genes in ESCC.

Key Words: Esophageal neoplasms, Carcinoma, squamous cell, p16 gene, p15 gene, Gene expression